In recent years, Europe etc., is advancing the adoption of offshore wind power generation, which employs a windmill installed offshore and performs power generation by utilizing force of wind, because of advantages of offshore areas, such as better wind state and smaller wind turbulence, as compared with onshore areas. In a case where electric powers generated by use of each of a plurality of windmills installed offshore are gathered, the following method is currently used in general: AC powers output from wind power generators, each of which is composed of a windmill and a power generator, are boosted by transformers and are collected, and then the collected electric power is further boosted by a transformer and is transmitted as a high voltage AC power to an onshore area.
Other than the above method, there has been proposed the following method: AC powers output from wind power generators are converted into DC powers by AC (Alternating Current)/DC (Direct Current) converters and are collected as the DC powers, and then the collected electric power is boosted by a DC/DC converter and is transmitted as a high voltage DC power to an onshore area. Since the number of transformers can be reduced because the collection is performed in the form of DC, there is provided an advantage in that the scale of each offshore electric power instrument can be smaller and the cost of each offshore construction can be thereby lowered. Further, since the power transmission to an onshore area is performed in the form of DC, the power transmission loss can be reduced.
In the case of the above system that collects electric powers in the form of DC, it is necessary to include DC breakers to remove accidents and to secure protection. Unlike the AC current, the DC current does not have a current zero point, and so the DC breakers need to form a current zero point in some manner.
As a manner of forming such a current zero point, for example, in a DC breaker disclosed in Patent Literature 1, a commutation circuit composed of a capacitor and a reactor is connected in parallel with a breaking part, and an electric charge, which has been pre-charged in the capacitor, is discharged to superpose a current, which is resonant with the reactor, onto a DC current, and thereby to form a current zero point.